The 3GPP Rel-13 feature “Licensed-Assisted Access” (LAA) allows LTE equipment to also operate in the unlicensed 5 GHz radio spectrum. The unlicensed 5 GHz spectrum is used as a complement to the licensed spectrum. An ongoing 3GPP Rel-14 work item adds UL transmissions to LAA. Accordingly, devices (i.e., LTE user equipment (UEs)) connect in the licensed spectrum (primary cell or PCell) and use carrier aggregation to benefit from additional transmission capacity in the unlicensed spectrum (secondary cell or SCell). Standalone operation of LTE in unlicensed spectrum is also possible and is under development by the MuLTEfire Alliance.
Regulatory requirements, however, may not permit transmissions in the unlicensed spectrum without prior channel sensing. Since the unlicensed spectrum must be shared with other radios of similar or dissimilar wireless technologies, a so-called listen-before-talk (LBT) method needs to be applied. LBT involves sensing the medium for a pre-defined minimum amount of time and backing off if the channel is busy. Today, the unlicensed 5 GHz spectrum is mainly used by equipment implementing the IEEE 802.11 Wireless Local Area Network (WLAN) standard, also known under its marketing brand as “Wi-Fi.”
Both Wi-Fi and LAA may operate in multi-carrier mode with simultaneous transmission across multiple unlicensed channels in the 5 GHz band. Wi-Fi follows a hierarchical multi-carrier LBT scheme known as channel bonding.
Long Term Evolution (LTE)
FIG. 1 illustrates the basic LTE downlink physical resource 100. LTE uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and Discrete Fourier Transform (DFT)-spread OFDM (also referred to as single-carrier FDMA (SC-FDMA)) in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid, where each resource element 110 corresponds to one OFDM subcarrier 115 during one OFDM symbol interval. The uplink subframe has the same subcarrier spacing as the downlink, and the same number of SC-FDMA symbols in the time domain as OFDM symbols in the downlink.
FIG. 2 illustrates the LTE time-domain structure 200. In the time domain, LTE downlink transmissions are organized into radio frames 210 of 10 ms, each radio frame 210 consisting of ten equally-sized subframes 215 of length Tsubframe=1 ms, in the illustrated example embodiment. Each subframe 215 comprises two slots of duration 0.5 ms each, and the slot numbering within a frame ranges from 0 to 19. For normal cyclic prefix, one subframe 215 consists of 14 OFDM symbols. The duration of each symbol is approximately 71.4 μs.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks (RBs), where a RB corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent RBs in time direction (1.0 ms) is known as a resource block pair. RBs are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled. For example, in each subframe, the base station transmits control information about which terminals data is transmitted to and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3, or 4 OFDM symbols in each subframe and the number n=1, 2, 3, or 4 is known as the Control Format Indicator (CFI). The downlink subframe also contains common reference symbols, which are known to the receiver and used for coherent demodulation of e.g., the control information. FIG. 3 illustrates an example downlink subframe 300 with CFI=3 OFDM symbols as control. The reference symbols shown there are the cell specific reference symbols (CRS) and are used to support multiple functions including fine time and frequency synchronization and channel estimation for certain transmission modes.
From LTE Rel-11 onwards, DL or UL resource assignments can also be scheduled on the enhanced Physical Downlink Control Channel (EPDCCH). By contrast, according to Rel-8 to Rel-10, only the Physical Downlink Control Channel (PDCCH) is available.
Physical Downlink Control Channel and Enhanced Physical Downlink Control Channel
The Physical Downlink Control Channel (PDCCH) and the Enhanced Physical Downlink Control Channel (EPDCCH) may be used to carry downlink control information (DCI) such as scheduling decisions and power-control commands. For example, DCI may include:                Downlink scheduling assignments, including Physical Downlink Shared Channel (PDSCH) resource indication, transport format, hybrid-Automatic Repeat Request (HARQ) information, and/or control information related to spatial multiplexing where applicable. A downlink scheduling assignment may also include a command for power control of the PUCCH used for transmission of HARQ acknowledgements in response to downlink scheduling assignments.        Uplink scheduling grants, including Physical Uplink Shared Channel (PUSCH) resource indication, transport format, and HARQ-related information. An uplink scheduling grant may also include a command for power control of the PUSCH.        Power-control commands for a set of terminals as a complement to the commands included in the scheduling assignments/grants.One PDCCH/EPDCCH may carry one DCI message containing one of the groups of information listed above. As multiple terminals can be scheduled simultaneously, and each terminal can be scheduled on both downlink and uplink simultaneously, there must be a possibility to transmit multiple scheduling messages within each subframe. Each scheduling message may be transmitted on separate PDCCH/EPDCCH resources, and consequently there are typically multiple simultaneous PDCCH/EPDCCH transmissions within each subframe in each cell. Furthermore, to support different radio-channel conditions, link adaptation can be used, where the code rate of the PDCCH/EPDCCH is selected by adapting the resource usage for the PDCCH/EPDCCH, to match the radio-channel conditions.Carrier Aggregation        
The LTE Rel-10 standard supports bandwidths larger than 20 MHz. One important requirement on LTE Rel-10 is to assure backward compatibility with LTE Rel-8. This should also include spectrum compatibility. That would imply that an LTE Rel-10 carrier, wider than 20 MHz, should appear as a number of LTE carriers to an LTE Rel-8 terminal. Each such carrier can be referred to as a Component Carrier (CC). In particular for early LTE Rel-10 deployments it can be expected that there will be a smaller number of LTE Rel-10-capable terminals compared to many LTE legacy terminals. Therefore, it is necessary to assure an efficient use of a wide carrier also for legacy terminals, i.e. that it is possible to implement carriers where legacy terminals can be scheduled in all parts of the wideband LTE Rel-10 carrier. The straightforward way to obtain this would be by means of carrier aggregation.
FIG. 4 illustrates aggregated bandwidth 400 by carrier aggregation (CA). CA implies that an LTE Rel-10 terminal can receive multiple CC 405A-E, where the CC have, or at least the possibility to have, the same structure as a Rel-8 carrier. A CA-capable UE is assigned a primary cell (PCell) which is always activated, and one or more secondary cells (SCells) which may be activated or deactivated dynamically.
The number of aggregated CC 405A-E as well as the bandwidth of the individual CC may be different for uplink and downlink. A symmetric configuration refers to the case where the number of CCs 405A-E in downlink and uplink is the same whereas an asymmetric configuration refers to the case that the number of CCs 405A-E is different. It is important to note that the number of CCs 405A-E configured in a cell may be different from the number of CCs 405A-E seen by a terminal. For example, a terminal may support more downlink CCs than uplink CCs, even though the cell is configured with the same number of uplink and downlink CCs.
In addition, a key feature of carrier aggregation in the ability to perform cross-carrier scheduling. This mechanism allows an EPDCCH on one CC to schedule data transmissions on another CC by means of a 3-bit Carrier Indicator Field (CIF) inserted at the beginning of the EPDCCH messages. For data transmissions on a given CC, a wireless device may expect to receive scheduling messages on the EPDCCH on just one CC—either the same CC, or a different CC via cross-carrier scheduling. The mapping from EPDCCH to PDSCH is also configured semi-statically.
Wireless Local Area Network
In typical deployments of WLAN, carrier sense multiple access with collision avoidance (CSMA/CA) is used for medium access. This means that the channel is sensed to perform a clear channel assessment (CCA), and a transmission is initiated only if the channel is declared as Idle. In case the channel is declared as Busy, the transmission is essentially deferred until the channel is deemed to be Idle.
When the range of several access points (APs) using the same frequency overlap, all transmissions related to one AP might be deferred in case a transmission on the same frequency to or from another AP within range can be detected. Effectively, this means that if several APs are within range, they will have to share the channel in time, and the throughput for the individual APs may be severely degraded. FIG. 5 illustrates an example listen before talk (LBT) mechanism 500 on a single unlicensed channel.
In the single-channel LBT case, after a Wi-Fi station A transmits a data frame to a station B, station B shall transmit the ACK frame back to station A with a delay of 16 μs. Such an ACK frame is transmitted by station B without performing a LBT operation. To prevent another station interfering with such an ACK frame transmission, a station shall defer for a duration of 34 μs (referred to as DIFS) after the channel is observed to be occupied before assessing again whether the channel is occupied.
Therefore, a station that wishes to transmit first performs a CCA by sensing the medium for a fixed duration DIFS. If the medium is idle then the station assumes that it may take ownership of the medium and begin a frame exchange sequence. If the medium is busy, the station waits for the medium to go idle, defers for DIFS, and waits for a further random backoff period.
To further prevent a station from occupying the channel continuously and thereby prevent other stations from accessing the channel, it is required for a station wishing to transmit again after a transmission is completed to perform a random backoff.
For multi-carrier operation, Wi-Fi follows a hierarchical channel bonding scheme to determine its transmission bandwidth for a frame, which could be 20 MHz, 40 MHz, 80 MHz, or 160 MHz for example. In the 5 GHz band, wider Wi-Fi channel widths of 40 MHz, 80 MHz, 160 MHz or 80+80 MHz are formed by combining 20 MHz sub-channels in a non-overlapping manner. A pre-determined primary channel performs the CW-based random access procedure after a defer period if necessary, and then counts down the random number generated. The secondary channels only perform a quick CCA check for a PIFS duration (generally 25 μs) before the potential start of transmission to determine if the additional secondary channels are available for transmission. Based on the results of the secondary CCA check, transmission is performed on the larger bandwidths; otherwise transmission falls back to smaller bandwidths. The Wi-Fi primary channel is always included in all transmissions, i.e., transmission on secondary channels alone is not allowed.
Load-based Clear Channel Assessment
For a device not utilizing the Wi-Fi protocol, Europe Regulation EN 301.893, v. 1.7.1 provides the certain requirements and minimum behavior for the load-based clear channel assessment. FIG. 6 illustrates an example LBT mechanism 600 in conformance with EN 301.893. The requirements and minimum behavior are as follows:                1. Before a transmission or a burst of transmissions 604 on an operating channel, the equipment shall perform a Clear Channel Assessment (CCA) 602 check by detecting the energy level of the operating channel. The equipment shall observe the operating channel(s) for the duration of the CCA observation time 606, which is set by the manufacturer and shall be not less than 20 μs. The Operating Channel shall be considered occupied if the energy level in the channel exceeds the threshold corresponding to the power level given in enumerated point #5 below. If the equipment finds the channel to be clear, it may send transmissions 604 immediately (see point #3 below).        2. If during CCA check 602, the equipment finds an Operating Channel occupied, it shall not transmit in that channel. The equipment shall perform an Extended CCA check 608 in which the Operating Channel is observed for the duration of a random factor N multiplied by the CCA observation time. N defines the number of clear idle slots 610 resulting in a total Idle Period that needs to be observed before initiation of the transmission. The value of N shall be randomly selected in the range 1 . . . q every time an Extended CCA 608 is required and the value stored in a counter. The value of q is selected by the manufacturer in the range 4 . . . 32. This selected value shall be declared by the manufacturer (see clause 5.3.1 q)). The counter is decremented every time a CCA slot is considered to be “unoccupied”. When the counter reaches zero, the equipment may transmit 612.                    It should be noted that the equipment is allowed to continue Short Control Signaling Transmissions on this channel providing it complies with the requirements in clause 4.9.2.3.            For equipment having simultaneous transmissions on multiple (adjacent or non-adjacent) operating channels, the equipment is allowed to continue transmissions on other operating channels providing the CCA check did not detect any signals on those channels.                        3. The total time that an equipment makes use of an operating channel is the maximum channel occupancy time 614 which shall be less than (13/32)×q ms, with q as defined in point #2 above. After the maximum channel occupancy time 614, the device shall perform the extended CCA 608 described in point #2 above.        4. Upon correct reception of a packet which was intended for the equipment, the equipment may skip CCA and immediately proceed with the transmission of management and control frames (e.g. ACK and Block ACK frames). A consecutive sequence of transmissions by the equipment, without it performing a new CCA, shall not exceed the maximum channel occupancy time as defined in point #3 above.                    For the purpose of multi-cast, the ACK transmissions (associated with the same data packet) of the individual devices are allowed to take place in a sequence.                        5. The energy detection threshold for the CCA shall be proportional to the maximum transmit power (PH) of the transmitter: for a 23 dBm e.i.r.p. transmitter the CCA threshold level (TL) shall be equal or lower than −73 dBm/MHz at the input to the receiver (assuming a 0 dBi receive antenna). For other transmit power levels, the CCA threshold level TL shall be calculated using the formula: TL=−73 dBm/MHz+23−PH (assuming a 0 dBi receive antenna and PH specified in dBm e.i.r.p.).Licensed-assisted access (LAA) to unlicensed spectrum using LTE        
Up to now, the spectrum used by LTE has been dedicated to LTE. This has the advantage that the LTE system does not need to care about the coexistence issue and the spectrum efficiency can be maximized. However, the spectrum allocated to LTE is limited, and the allocated spectrum cannot meet the ever increasing demand for larger throughput from applications and/or services. Therefore, a new work item has been initiated in 3GPP on extending LTE to exploit unlicensed spectrum in addition to licensed spectrum.
FIG. 7 illustrates licensed-assisted access (LAA) to unlicensed spectrum using LTE carrier aggregation. As depicted, a wireless device 710 is connected to a primary cell (PCell) 712 in the licensed band and one or more secondary cells (SCells) 714 in the unlicensed ban. Herein, a secondary cell in unlicensed spectrum may be referred to as a LAA secondary cell (LAA SCell). The LAA SCell may operate in downlink only mode or operate with both UL and DL traffic. Furthermore, certain embodiments may include LTE nodes operating in standalone mode in license-exempt channels without assistance from a licensed cell. Unlicensed spectrum can, by definition, be simultaneously used by multiple different technologies. Therefore, LTE needs to consider the coexistence issue with other systems such as IEEE 802.11 (Wi-Fi).
For LAA, the backoff counter does not have to be decremented when a slot is sensed to be idle during the extended clear channel assessment (ECCA) procedure. Additionally, any subframe can be used for either DL or UL transmission.
To coexist fairly with the Wi-Fi system, transmission on the SCell must conform to LBT protocols in order to avoid collisions and causing interference to on-going transmissions. This includes both performing LBT before commencing transmissions, limiting the maximum duration of a single transmission burst, and limiting transmit power. The maximum transmission burst duration is specified by country and region-specific relations, e.g., 4 ms in Japan and 13 ms according to EN 301.893.
FIG. 8 illustrates LAA to the unlicensed spectrum with LBT and UL and DL transmissions within a transmission opportunity (TXOP). Specifically, in the example depicted, a 4 ms LAA TXOP 802 after successful LBT 804 consists of a DL transmission burst 806 with two subframes followed by an UL transmission burst 808 of two subframes. Thus, there is TXOP sharing between the downlink and the uplink. The UL burst 808 may perform a single CCA, a short extended CCA, or no CCA before transmission.
Multi-Carrier Operation
The use of LTE CA, introduced since Rel-10, offers means to increase the peak data rate, system capacity and user experience by aggregating radio resources from multiple carriers that may reside in the same band or different band.
In Rel-13, LAA has attracted a lot of interest in extending the LTE CA feature towards capturing the spectrum opportunities of unlicensed spectrum in the 5 GHz band. WLAN operating in the 5 GHz band already supports 80 MHz in the field and 160 GHz is to follow in a second wave deployment of IEEE 802.11ac. Enabling the utilization of multi-carrier operation on unlicensed carrier using LAA is deemed necessary as further CA enhancements. The extension of the CA framework beyond five carriers has been started in LTE Rel-13. The objective is to support up to 32 carriers in both UL and DL.
For LAA, the eNB is allowed to transmit DL data burst(s) on the carriers that have completed ECCA with potential self-deferral (including idle sensing for a single interval) to align transmission over multiple carriers. A point under further study in LAA is that if the network node can receive on a carrier while transmitting on another carrier, backoff counters for the carriers not transmitting while other carriers are transmitting may be frozen if the carriers are within X MHz of each other. However, the value of X has yet to be determined.
LTE systems currently allow licensed carriers to be aggregated and utilized for data transmission to boost the throughput. Due to the introduction of LAA in 3GPP Rel-13, there is a need to support multi-carrier operation on unlicensed carriers. Hence, the LBT design should be carefully considered for multi-carrier operation.
One existing solution is that the network node does LBT for each carrier on unlicensed spectrum in order to access the channel. If the LBT succeeds on one carrier, the network node transmits on this carrier. Furthermore it is generally not possible to transmit on one LAA carrier and simultaneously receive or sense the channel on adjacent carriers due to adjacent channel interference. For example, consider a LAA TXOP consisting of a DL transmission burst followed by an UL transmission burst, i.e., TXOP sharing between DL and UL. FIG. 9 illustrates two LAA SCells 902A-B wherein an UL burst 904 is interfered by a DL burst 906 on an adjacent carrier with autonomous sensing.
If each carrier such as LAA SCell 902B performs autonomous sensing such as energy detection and defers LBT 908 while an adjacent carrier is transmitting on the DL, a carrier may resume LBT 910 and start transmitting during the UL portion 904 of the adjacent carrier's TXOP due to the lack of adjacent channel leakage energy. This UL portion 904 will then not be decodable due to the sudden DL transmission 906 of the adjacent carrier, leading to an inefficient use of the unlicensed spectrum.